1. Field of the Invention
This invention relates to an electrophotographic photosensitive member having a specific photosensitive layer (inclusive of a charge generation layer and a charge transport layer) and a reflecting layer, and a process cartridge and an image forming apparatus having the electrophotographic photosensitive member.
2. Description of Related Art
In recent years, in order to improve the quality of images output from electrophotographic apparatus, development directed to higher resolution has been increasingly promoted. Measures applied to the apparatus for this end are relatively easy from the aspect of optics. More specifically, an increase in resolution can be achieved by reducing the spot diameter of a laser beam and increasing writing density. However, with semiconductor lasers of approximately from 630 nm to 780 nm in lasing wavelength (emission wavelength) which are conventionally used as light sources, it has been found that the sharpness of spot contours is difficult to attain in some cases even if the spot diameter of a laser beam is reduced by operation of an optical system. This is due to the diffraction limit of a laser beam, and is for the reason that the lower limit (D) of the spot diameter is directly proportional to the emission wavelength λ of a laser beam, as represented by the following equation:D=1.22λ/NA
wherein NA represents the numerical aperture of a lens.
In electrophotographic processes, red laser beams conventionally commonly used as image exposure light have emission wavelengths as long as approximately from 630 to 780 nm. Hence, as is clear from the above equation, it is difficult to reduce the beam spot diameter beyond a certain extent. This brings about a problem in that the recording density on a photosensitive member can not be increased beyond a certain extent. To cope with this problem, it is necessary to shorten the lasing wavelength of a semiconductor laser.
To shorten lasing wavelengths of lasers, some techniques have been known in the art.
One of them is a technique in which a non-linear optical material is utilized and the emission wavelength of a laser beam is halved by second harmonic generation (SHG) (see, e.g., Japanese Patent Applications Laid-open No. H09-275242, No. H09-189930 and No. H05-313033). In this technique, GaAs LDs and YAG lasers, which have been already established as a technique capable of emitting high-power light, can be used as a primary light source, hence long life and high output can be ensured.
Another is a technique in which a wide-gap semiconductor is used, and an apparatus can be miniaturized as compared with devices using the SHG. LDs using ZnSe semiconductors (see, e.g., Japanese Patent Applications Laid-open No. H07-321409 and No. H06-334272) or LDs using GaN semiconductors (see, e.g., Japanese Patent Applications Laid-open No. H08-88441 and No. H07-335975) have been subjects of research for a long time for the reason that they are high in emission efficiency.
However, it is difficult for these LDs to be optimized in respect of their device structure, crystal-growth conditions, electrodes and so forth, and because of defects or the like in crystals, it has been difficult to perform long-time oscillation at room temperature, which is essential for them to be put into practical use. However, with progress of technological innovation on substrates and so forth, Nichia Corporation reported in October, 1997, that an LD using a GaN semiconductor allowed for continuous oscillation for 1,150 hours (at 50° C.), and started to sell the LD from October, 1997.
Where latent images are formed on a multi-layer type electrophotographic photosensitive member by using laser beams, there has been such a problem that interference fringes tend to appear when the photosensitive member has a charge generation layer having small absorbance at the emission wavelengths of the laser beams. However, in order to increase the absorbance, if the charge generation layer is made too thick, it tends to lower dark-area potential or bring about ghosts. Accordingly, as a method for resolving such a problem, it is proposed that a reflecting layer having the function of erasing the coherence of laser beams is provided between a support and a photosensitive layer (see Japanese Patent No. 2502286).